1. Field of the Invention
The present invention relates generally to electrophotographic imaging device and, more particularly, to fusers of electrophotographic imaging devices.
2. Description of the Related Art
In the electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image, and thereafter the toner image is transferred to the media intended to receive the final permanent image. The toner is fixed to the media by the application of heat and pressure in a fuser. A fuser may include a heated roll and a backup roll forming a fusing nip through which media passes, known as a hot roll fuser. A fuser may also include a fuser belt and an opposing backup member, such as a backup roll, known as a belt fuser.
A hot roll fuser is a high force and pressure fuser that can deliver high print quality, however a hot roll fuser is not an “instant on” fuser having a very short warm-up time due to the huge thermal mass of thick metal core and thick silicone rubber layer coated on the metal core. While a belt fuser with a ceramic heater or induction heater can be an “instant on” fuser, it is usually only used for low speed color laser printers due to lower belt operating temperatures. Since the belt is only heated within a fuser nip region, belt temperature during a print operation is about 60 degrees C. lower than its heater temperature. Therefore, a belt fuser with a ceramic heater has difficulty to provide enough heat for high speed fusing. In addition, the fusing quality of a belt fuser with a ceramic heater is not as good as that of a hot roll fuser.
In order to achieve a very short warm-up time, an “instant on” fuser, like a belt fuser with a ceramic or induction heater, uses an endless fusing belt that can be heated very fast due to its small thermal mass. Since the fusing belt is very thin and flexible, force cannot be directly applied to both ends of the belt to form a required fuser nip. To form a fuser nip, a stationary pressure member, a heater and a heater housing with a steel bracket for a ceramic belt fuser is put inside the belt tube. Forces are applied to both ends of the steel bracket and the pressure member forces the fusing belt to firmly contact against a backup roll to form a fuser nip. The pressure member is fixed and does not turn. Since the pressure member is not turning with the belt, friction forces between the contact surfaces of the belt and the pressure member are very high and can wear the belt and ceramic heater, which reduces fuser lifetime. Even with lubrication between the contact surface of the belt and the stationary pressure member, belt stall still occurs as the lubrication often fails which causes serious wear on heater surface or belt. In order to reduce the friction force, the force used for forming a fusing nip has to be much lower than the force applied to a hot roll fuser. The lower force results in lower nip pressure and the lower nip pressure can cause many print quality problems, such as poor fuse grade, mottling, poor uniformity across a page, and transparency defects.
Thus, there is still a need for a fuser with fast warm up time and high nip pressure in order to deliver high print quality.